1. Field of the Invention
The present invention pertains to wireless telecommunications, and particularly to alignment of plural transmit paths in a radio base station node of a telecommunications system.
2. Related Art and Other Considerations
In a typical cellular radio system, mobile user equipment units (UEs) communicate via a radio access network (RAN) to one or more core networks. The user equipment units (UEs) can be mobile stations such as mobile telephones (“cellular” telephones) and laptops with mobile termination, and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network.
The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a (radio) base station. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by a unique identity, which is broadcast in the cell. The radio base stations communicate over the air interface (e.g., radio frequencies) with the user equipment units (UE) within range of the base stations. In the radio access network, several radio base stations are typically connected (e.g., by landlines or microwave) to a radio network controller (RNC). The radio network controller, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural radio base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
One example of a radio access network is the Universal Mobile Telecommunication System (UMTS) Terrestrial Radio Access Network (UTRAN). The UMTS is a third generation system which in some respects builds upon the radio access technology known as Global System for Mobile communications (GSM) developed in Europe. UTRAN is essentially a radio access network providing wideband code division multiple access (WCDMA) to user equipment units (UEs). The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM-based radio access network technologies.
For sake of providing diversity, some radio base stations have plural RF transmission paths between a baseband transmitter and a radio base station antenna. In such cases, and particularly in a WCDMA system, significant constraints are imposed on the operation of the radio base station regarding the allowed timing difference of transmit (TX) transmission on the different transmission paths. For example, in accordance with one 3GPP technical specification, the allowed deviation between transmission paths is ⅛ chip, which corresponds to approximately 32 ns.
The technology currently used in radio base station nodes which operate in accordance with GSM or WCDMA is to statically calibrate the delay (e.g. on the transmit paths). This static delay calibration occurs e.g. during start-up of the system. The delay values used in the calibration are either general design based figures or explicitly determined by production measurements for hardware entities.
It has also elsewhere been proposed to use test mobiles or other type of radio based receivers for calibration and adjustment purposes related to transmit delay and time alignment. For example, see U.S. patent application Ser. No. 10/045,024, filed Jan. 15, 2002, entitled “DIVERSITY BRANCH DELAY ALIGNMENT IN RADIO BASE STATION,” which is incorporated herein by reference.
It is extremely onerous to comply with the requirements concerning the allowed timing difference of transmit (TX) transmission on the different transmission paths. Difficulties arise from various factors or timing uncertainties and/or variations in the radio base station nodes themselves, both in digital components and (perhaps even more) in analog components forming a transmit path up to the antenna. The problems of delay differences and variations are particularly acute in the case of co-sited radio base stations (occurring, e.g., in GSM and UMTS) where radio frequency (RF) equipment is shared.
What is needed, therefore, and an object of the present invention, is technique, apparatus, and method for providing more accurate alignment timing between differing transmit branches of a radio base station node.